Process for producing fluorides



Patented Mar. 20, 194 I Ul'iED SATES "Arum OFFlCE PROCESS FOR PRODUCiN G FLUORIDES Donald A. Swalheim, Cleveland, Ohio, assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. A ppllcation'February 9, 1944, Serial No. 521,684

2 Claims. (Cl. 23-48) This invention relates to methods for reducing plex compound of stannous fluoride and sodium the tin content of stannates to stannous form, fluoride, but this complex is readily soluble in and is more particularly related to processes in the bath and its use presents no difficulty.

which an alkali flu-ostannate is converted to a In carrying out a process of my invention, I stannous fluoride and an alkali fluoride by effect- 5 prefer to use the sodium fluostannate in a dry ing contact of the fluostannate with molten tin, form since danger isinvolved if there is moisture preferably in the presence of residual stannous present in the material added to the molten tin. fluoride. Themolten tin may advantageously be contained -In aqueous solutions for the electrodeposition in a nonmetallic receptacle, such as a graphite of tin, it is desirable that the tin be in stannous crucible. The fluostannate may be added in any form in order to achieve a maximum current efsuitable manner, but it is preferred that the adflciency. An excellent tin electrodepositing soludition be gradual and that the tin and fiuostantion may, for instance, contain stannous fluoride nate be mixed thoroughly after each addition.

and preferably also sodium fluoride. Unfortu- Continuous mixing in combination with continnately, in the large-scale use of such a solution 5 uous addition of fluostannate gives the best'reas, for instance, in a continuous strip plater for sults.

depositing the tin on sheet steel, it is practically The tin should be maintained in a molten con- -impossible to avoid a certain amount of oxidadition at all times, but it is preferred to have tion of the stannous salts present. It is necesthe temperature not substantially higher than is sary, for instance, in most strip platers to aglnecessary to maintain this mBlten condition. Actate the solution at rapid rate, and while the count must be takenof the cooling effect of the rate of oxidation may e minimized by excluding foregoing stannate additions. I have found that air as much as possible during such agitation, ordinarily excellent results may be obtained by the oxidation cannot be prevented entirely maintaining the tin in a temperature range of The result of the oxidation in a sodium fluoabout from 300 to 500 C. rides stannous salt bath is the formation of a The fluostannate should be allowed to remain sludge of sodium fluostannate. While this sludge in contact with the molten tin for a time sumdoes not seriously interfere with the operation of cient to reduce the stannic tin to stannous form. the bath, nevertheless, the tin in the sludge is'no Once intimate mixture of the fluostannate. and longer available for electrodeposition and evenmolten tin has been obtained, the reduction pro-- tually the sludge must be removed for best opceeds rapidly and is usually complete ina few eration of the bath. Ordinarily, the removal of minutes. the fluostannate sludge presents no dimculty, The proportions of fluostannate and tin used since it may be settled out or filtered out of the may be widely varied, depending on the manner bath. The recovery of the tin content of the soof operation, but in an eventthe tin should dium fluostannate baths, however, presented a v be at least stoichiometrically ,equivalentto' the difficult problem. fluostannate; that is, one mol of tin for one -mo1 Now I have found that if sodium fluostannate of fiuostannate. It. may be desirable in some is brought into contact with molten tin, prefercircumstances to maintain a residue or heel of ably in the presence of stannous fluoride, the 40 molten tin, tapping the reduced product off the stannic tin is reduced to the stannous form and top of the residue.

there is obtained a mixture of stannous fluoride The nature of my invention and its manner of and sodium fluoride which may be reused to application may be more readily understood by advantage in sodium fluoride-stannous salt elec-'- reference to the following illustrative example.

troplatlng solutions. as E l By the use of a process of this invention, the e entire tin content of a sodium fluostannate I This sample was carried out in a graphite crusludge, such as results from oxidation in a sodicible heated by gas and fitted with a tap tube um fluoride-stannous salt electroplating bath, about 3 inches above the bottom forremoval of.

may be completely recovered in a form suitable the reduced material. for adding directly to the bath.' The bath thus Two pounds of molten tin was placed in the becomes self-sustainin ithat is, the salts recovcrucible, melted, and maintained at a temperaered fromthe sludge replace electrolyte lost by ture in the range of 300 to 500 C. Two pounds drag out from the solution. The products of my of sodium fluostannate, obtained as a sludge from recovery processes may be in the form of a coma sodium fluoride-stannous salt electrodepositing bath and dried at a temperature of 100 C., was added to the tin in small increments over a period of one hour with periodic stirring to insure contact between the molten tin and the sludge. The reaction products were very fluid and were removed through the tap tube. The reduced product was found to be readily soluble in water and by analysis was found to contain the following:

While in the foregoing description of this invention, I have shown certain specific processes. it should be understood that without departing from the scope of my invention, one skilled in the art may employ numerous processes for reducing sodium fiuostannate with molten tin.

1. In a process for converting the tin content of sodium fluostannate to the stannous form, the step comprising bringing the sodium 'fluostannate into contact with molten tin.

2. In a process for converting the tin content of sodium fluostannate to the stannous form, the step comprising bringing the "sodium fluostannate into contact with molten tin in the presence is of stannous? fluoride.

DONALD A. 

